In mammals, the permanence of hearing loss is due mostly to the incapacity of the cochlea to replace lost mechano-receptor cells (i.e., hair cells [HCs]). The generation of new HCs from a renewable source of progenitors is a principal requirement for developing a cell therapy within this sensory organ. A subset of stem cells, termed side population (SP), has been identified in several tissues of mammals. The ATP-binding cassette transporter Abcg2/Bcrp1 contributes to the specification of the SP phenotype and is proposed as a universal marker for stem/progenitor cells. A defining character of these SP cells is a high efflux capacity for Hoechst dye. Here, we demonstrate that Abcg2 transporter is expressed with two other stem/progenitor cell markers (i.e., Nestin and Musashi1) in distinct and overlapping domains of the supporting cells within the postnatal cochlea. We have developed and describe a fluorescence-activated cell sorting (FACS) technique that enables the purification of a discrete subpopulation of SP-supporting cells from the early postnatal mouse cochlea based on their ability to exclude Hoechst dye. These FACS-isolated cells can divide and express markers of stem/progenitor cells such as Abcg2, a determinant of the SP phenotype, and Musashi1, a neural stem/progenitor cell marker. These markers can differentiate cells expressing markers of HCs and supporting cells in vitro. Our observation that these SP cells are capable of differentiating into HC-like cells implies a possible use for such cells (i.e., the replacement of lost auditory HCs within damaged cochlea).